Strained hip flexor?

Background

Last night when stretching before my BJJ class my left hip (flexor?)(very top of my leg where it meets your pelvis area) felt a little tight, this is unusual as I'm quite flexible, stretched out and finished up class. I've got kickboxing straight after and halfway through I was noticing it more, it was getting "tighter" and a fair bit sorer.

I should have stopped but at the time wasn't too bad, ended up working through it, went home, after an hour or so and was feeling it quite a bit, iced it up a little, and attempted to sleep it off, that didn't work so well, anytime I moved it woke me up, suffice to say, in the morning it was really caining.

I can raise my leg (straight legged from the pelvis) backwards, and sideways, when I try to lift directly upwards in front it feels as if I've got no strength and can't lift it close to 90 degrees without side bending at the waist slightly, it's quite sore, feels much like a muscle cramp in the top of my thigh.

There was no incident that I'm aware of that caused it.

I can still stretch out to full ROM, with only slight soreness, but trying to use (push my hips out or move left leg is a killer)

Question

How do you rehab something like this? Do more stretches? (I googled, and got conflicting advice re: stretching to rehab hip flexors)

Using some Voltaran on it at the moment

Do I just R.I.C.E it? Anyone that has experienced this give me a rough timeframe? Should I lay off BJJ/KB and the Gym? Not really looking forward to missing out on training.